Today's tourists often need wading boots to explore the architectural wonders of Venice. Will they one day need scuba gear? NOVA covers the battle to keep the world's most unusual city from drowning beneath the rising tides of the Adriatic Sea. The lessons we learn about how to stop rising sea levels will prove essential for other coastal cities around the world, from New York to Shanghai.

For centuries, Venetians have been fighting the forces of nature that threaten to alter their city's precarious relationship with the encircling lagoon that has long served as protection from invading armies. In the 15th century, the trouble was silt, which was filling up the lagoon from nearby rivers. Environmental engineers of the day solved this problem at enormous expense by diverting the rivers to more distant outlets.

More recently the peril has been the subsidence of the ground on which Venice is built, combined with ever higher tides caused by rising sea levels. When storm winds blow from the south, the Adriatic floods the lagoon, causing acqua alta, or high water. Under certain weather conditions, the flooding can be catastrophic. Venetians have kept up with the rising water by raising the level of floors and pavement. This has deformed elegant buildings, created awkward doorways, and left very little room to keep building up in advance of acqua alta.

Another alarming sign is the green ring of algae at the high-water mark. Ominously, this line has crept above the impermeable foundation stones that have long kept saltwater from seeping into walls, slowly destroying buildings from within. Infrared images show that the saltwater is indeed inside doing its damage.

Venetians, however, are not without a plan to save their city. Engineers have proposed a multibillion-dollar series of gates that will rise off the seafloor at the entrances of the lagoon whenever acqua alta is forecast, holding the sea back until the high tides subside (Watch a video about the proposed gates).

Not everyone supports the gate project. American archeologist Albert Ammerman recently made headlines when he announced his conclusion that Venice is sinking faster than previously believed. Ammerman and others also criticize the planners of the gates for not taking into account the worst-case scenarios for sea-level rise caused by global warming. If the sea rises faster than the engineers predict, frequent gate closings could severely inhibit water circulation in the lagoon, turning the lagoon into a cesspool of industrial waste, agricultural runoff, and raw sewage, all of which now flow out to sea with the tides.

If Ammerman is right, the working life of the gates may be short. "The people who are claiming that this is going to be a successful solution for 100 years, 200 years have no basis for that whatsoever, in terms of the best guesses about global warming," worries Ammerman. He believes more studies should be done on the impact of the gates and that it may be necessary to go back to the drawing board to find a more viable long-term solution.

But supporters of the gates counter that rapidly rising sea levels mean the need for the gates is more urgent than ever. "I think you have to act and learn simultaneously," says environmental studies expert Pier Vellinga. "We think this solution is very good for the short term. And if you postpone and study and study and study, we may study until the sea has risen 50 centimeters [20 inches]. Then you're not safe. If you have the barrier in place, you have insurance for the future."

In December 2001, the Italian government announced that it would indeed build the gates, though as John Keahey points out in Weighing the Solutions, Venetian leaders remain ambivalent about the gates, and the current pro-gates national government might be replaced by one not so supportive of the idea. So whether the gates will actually be built remains to be seen. In the meantime, the seas continue to rise.

Sea of trouble: Historic St. Mark's Square is awash about a third of every year.